The invention relates to the telecommunications field and particularly to switching architectures which use serial telecommunication links transporting 8B/10B coding.
Patent applications 96480126.0, 96480125.2, 96480117.9, 96480120.3 are non-published European applications illustrating a powerful self-routing switch that provides a high switch rate.
Modem switching architectures requires powerful switching structures and distributed Switch Core Access Layer (SCAL) elements that provide the attachment of the telecommunications lines. Such an architecture can be found in the copending application filed on the priority date of the present application, and entitled xe2x80x9cFlow Control Process for a Switching System and System for Performing the Samexe2x80x9d and assigned to the assignee of the present application, which is herein incorporated by simple reference.
Since the central switching structure has some limited switching capacity it may obviously happen that, on one or more input ports, the switching structures require reducing the incoming flow of data that arrives in the port. Obviously this is achieved by means of a special channel that provides the flow control information for that purpose.
However since the distance between the switching structure and the different distributed entities (SCAL elements) that can be located in different premises within an industrial area tends to increase, even attaining separations some hundreds of meters, it is highly desirable that the flow control channel does not use any additional physical media. Known techniques to provide such flow control channels may use cell overhead by dedicating some of the bits that are conveyed to this particular flow control information. However, this results in waste of the useful bandwith.
This requirement is even more critical when the switching architecture is based on a port expansion mode, resulting in a great number of individual switches being connected.
The problem to be solved by the present invention is to provide an improved flow control channel for high speed switching architectures based on a centralized switching structure and distributed Switch Core Access Layers (SCAL) entities that does not require additional physical media or use a minimum part of the available bandwith.
It is an object of the present invention to provide a flow control channel for a switching architecture which can be used even when it is mounted in a port expansion mode.
This problem is solved by the flow control channel of the present invention which uses two among the three available xe2x80x9ccomma charactersxe2x80x9d in order to create this specific flow control channel. When the cells are idle or empty, the nature of the comma character that appears at the beginning of the cell provides the appropriate flow control bit information. For instance, should the K.28.5 character be detected, the receiving entity (either the switching structure or a distributed SCAL element) shall decode the latter as positive flow control information, corresponding for instance to a request to reduce the incoming data flow. Also, should the K.28.1 character be decoded, then the receiving entity shall decode it as information according to which no reduction in the data flow is requested. When the incoming flow provides data cells, the invention uses a predetermined bit within the data cell, generally that immediately following the beginning of the cell, in order to carry the flow control information.
Preferably, the invention uses a second channel which is also based on the empty cells and which is used for transporting quasi-static information that does not require immediate transport. This provides an effective combination of two distinct control channels, a first cell frequency channel such as defined above (providing a one-bit flow control channel at the cell rate) and a second quasi-static control channel for control information that does not need immediate transport or for which real time requirements are less important.
The invention can be improved by using an additional specific mechanism that monitors, at the transmitting point, the change of one control information which is to be transported through the quasi-static control channel. From the instant of the change, the mechanism initiates a counting process that automatically launches, if necessary, the creation of an empty cell so that the change of the considered control information can be transmitted and reported to the other side of the serial link within a maximum specified delay (also called edge distortion).
The invention also provides a switching system having an effective flow control mechanism.